1. Technical Field
The disclosure relates to passive infrared sensor-based motion detectors.
2. General Description
Motion detectors based on infrared sensing elements represent a specialized type of passive infrared (PIR) sensor called Passive Infrared-based Motion Detectors (PID). PIDs are routinely applied to security systems and for automatically actuated lighting systems which are intended to be triggered by movement of an object or body through the sensor's coverage area. PIDs do not literally detect “motion”, but rather detect transient changes in temperature occurring within usually small and changing portions of the coverage area. These transient changes, or “apparent motion”, can stem from movement of a warm bodied person or vehicle into or through the coverage area, producing a local area of higher temperature against the cooler background of the coverage area. Such transient changes can also be associated with movement of a body or an object colder than the background through the coverage area. An example of this would be ice floating on a warmer river.
A PID is usually built using a plurality of pyroelectric thin film sensors which are combined with appropriate circuitry to make the PID insensitive to changes in ambient temperature generally detected across the coverage area while remaining sensitive to the localized temperature changes likely to be associated with movement of objects of a different temperature in or through the coverage area. Ambient temperature changes generally occur more slowly and across more of the coverage area than changes observed upon the movement of objects through the area allowing generation of signals generally indicative of object movement based on transient localized changes in temperature in the coverage area. In a literal sense then, PID motion sensors could be considered “apparent” motion detectors since the changes they detect are not exclusively caused by motion of objects, however, the term “motion detector” is commonly used in the art.
Unfortunately, transient changes can also stem from events which have little or nothing to do with the movement of “objects of interest”. Handling of localized, transient changes of temperature which are not the result of motion of an object of interest, but are instead associated with other events, for example wind induced motion of tree branches or changes in cloud cover, is an issue for PID based systems. Unusually large objects, or objects exhibiting a substantial deviation from ambient temperature may pass through peripheral portions of the coverage area (or just outside the intended coverage area) giving rise to undesirable triggering of lighting or security alerts (“false” triggers).
The sensor elements in PIDs produce what is basically an analogue signal subject to variation over time depending upon changes in the heat output, or reflection, within its field of view. Signal events must be accurately correlated with the motion of warm bodies of significant size moving into or out of the focused sensing zones. A warm person walking into a sensing zone can generate a relatively large change in the infrared energy in that sensing zone, causing the circuitry to interpret that change in infrared energy as motion. One difficulty a sensor has is distinguishing between signals which indicate object motion in the coverage area, in which case it is desirable to activate the lights, and other signals which are not caused by object motion but mimic the signal resulting from object motion and thus which result in activation of security lights. These false triggers or nuisance activations need to be minimized. Minimization of false trigger events has demanded careful aiming of PIDs if maximum sensitivity is to be employed. As a consequence, less than maximum PID sensitivity is sometimes used with less than optimal aiming of the device as a tradeoff to minimize installation time.
Prior art PIDs have been built which have included a secondary light which is activated whenever a signal is generated which is of sufficient magnitude to trigger activation of the primary lighting or to activate an alarm. In some applications the secondary light is activated for a relatively brief period compared to the primary lighting. The secondary light provides several advantages. One advantage is that initial installation is aided since an installer can determine the sensor coverage area by watching the secondary light turn on and off. The “on period of the secondary light is generally much shorter than the “on” period for the primary lights and often corresponds closely to when an object is actually in the coverage zone as opposed to the main lights which may have an “on” period of several minutes. Another advantage is that the secondary light can be left active during daylight hours when the primary lighting is inoperable to save power. Still another advantage is that the secondary light can serve as an indicator that the primary lighting has failed.
Prior art PIDs have often been used as part of an area security system for detecting intruders within a secured area. Upon detection of an intruder the primary lighting or an alarm is activated. A secondary light, if present, also turns when the intruder enters the secured area. While the primary lighting typically remains on for an extended period of time, for example five minutes, the secondary light typically turns off after just a few seconds and then, if additional object movement is detected within the coverage area, turns on again. The secondary light can flash on and off if the intruder moves around in or further into the coverage/secured area. However, intruders often vacate a secured area in response to activation of the primary lighting, particularly if the intrusion is innocent.
While PIDs have been built to accommodate ambient conditions, changes in ambient conditions can still affect the precise borders of a PID's coverage area and/or the minimum temperature differentials between an object and its background required to activate the primary lighting or alarm. False triggers, or failures to respond, may be more or less likely when the weather does not match the conditions prevailing on installation. Small changes in ambient conditions may cause signals that were just below the threshold necessary to activate the primary lights or alarm. A typical example would be a PID installed at the front of a house and inadvertently aimed so that traffic passing in front of the house was within its field of view but far enough away that the signals generated by passing cars were not large enough to activate the system's primary response. Should a vehicle later exhibit a substantial temperature contrast with the environment, often stemming from colder weather conditions, the signal may become large enough to trigger the system's primary and secondary response.